You're gonna have find some way to find the correct dimensions for the
rockers and the whole floor area.
One of the big problems with body-on-frame cars is the body has no real
stiffness, unlike a unibody which can be made so it hardly moves when panels
are cut out.
Can you find a similar car ( a same year B-O-P body should be close) and get
the dimensions you need? Then jack it back to shape and weld in your braces.
Have you found a source for your panels? If it's a well reputed source, like
Year One, then the panels themselves can help in fixing things up.
John 3:16-17, Ephesians 2:8-9, Romans 3:23, 6:23
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